A local positioning system (“LPS”) consists of actuators and a motion controller, and is used to measure and indicate locations of points in the local coordinate system of the target object. An LPS may be used to indicate (e.g., with a laser spot) an aim point on a target object. The aim point is the intersection of the device aim axis and the surface of the target object. A typical LPS may include a camera and a laser-based range meter mounted on a 2-axis, motorized pan-tilt unit. The pan-tilt unit may be controlled by a computer in either interactive control mode, wherein the user instructs the LPS where to point using an input device (e.g., a multi-axis joystick), or automated position playback mode, wherein the computer instructs the LPS where to point based upon stored data.
A user may receive visual feedback from the LPS by observing the movement of the aim point (e.g., the laser spot) as it moves from target location to target location across the target object. However, difficulty controlling the LPS in the local coordinate system may arise when the requested motion requires an aim direction of the LPS that is not perpendicular to the surface of the target object—a common situation, as those skilled in the art will appreciate. In such a situation, the direction and speed of motion of the aim point will not be in the exact direction and speed that the user specified.
For example, as shown in FIG. 1, a local positioning device 10 may align its aiming axis by projecting a laser beam 11, which terminates in a laser spot 12 (i.e., the aim point) on a target object 14 having a local coordinate system 16. In this example, the local positioning device 10 includes an articulation mechanism having two intersecting rotational actuators with internal motion specified in terms of pan and tilt angles. An input command to the local positioning device 10 to draw a straight line across the x-axis of the target object 14 may not yield straight, horizontal line motion from the point of view of the local positioning device 10, but rather may result in an arc-shaped path with mostly horizontal motion and some amount of positive or negative vertical motion, depending on the aim axis intersection angle and on the type of articulation mechanism used to position the beam 11. For the type of local positioning device shown, the result is a perceived curved line 18 being drawn across the target object 14 as the local positioning device 10 moves the projected aiming axis from one target location to another target location. Furthermore, the local velocity (as measured in target coordinates) of the aim point laser spot 12 moving across the target object 14 may not be constant throughout the range of motion, but rather may increase at a non-linear rate as the laser spot 12 gets further away from perpendicular alignment with the target object 14.
As another example, as shown in FIG. 2, the local positioning device 10 may be instructed to trace a circle on the target object 14′ by moving the laser spot 12′. Using inverse kinematics alone, the process of tracing the circle involves making a sequence of points on the perimeter of the desired shape, thereby resulting in a piecewise linear path 20 that only somewhat resembles a circle. Indeed, multi-segment paths are required even for a simple straight line shape since the motion between the points will not be linear due to the rotation-based motion of the point-to-point control of the pan-tilt device. This requires a large number of positions to produce a close approximation of the shape. Therefore, in the automated position playback mode, the application software sends a Move-To command for each point in which the pan-tilt device comes to stop at each point, which causes the process of tracing shapes to be too slow for practical application in many of the intended use cases (such as area identification). In addition, multi-segment path with a large number of starts and stops may lead to excessive ware and tear on the device's components.
Accordingly, those skilled in the art continue to seek advances in the field of local positioning systems, including obtaining motion in the target coordinate system that reflects the intentions of the user such that interactive control mode and automated position playback mode would be improved.